Cellulose acylate films have conventionally been used for photographic supports or various optical materials owing to their toughness and flame retardancy. In recent years, they are frequently used as an optical transparent film used for liquid crystal display devices. Cellulose acylate films are excellent as an optical material for devices treating polarized light such as liquid crystal display device because they have optically high transparency and optically high isotropy. They have therefore been used so far as a protective film of a polarizer or a support of an optically-compensatory film capable of improving (compensating a viewing angle of) the display viewed from a diagonal direction.
In recent liquid crystal display devices, there is a strong demand for the improvement of viewing angle characteristics. An optically transparent film such as a protective film of a polarizer or support of an optically-compensatory film is required to be optically isotropic. For optical isotropy, a small retardation value of an optical film expressed by the product of a birefringence and a thickness is important. Above all, in order to improve the display from a diagonal direction, it is necessary to reduce not only the in-plane retardation (Re) but also retardation (Rth) in a thickness-direction. More specifically, when the optical properties of an optically transparent film are evaluated, Re as measured from the frontal side of the film is required to be small and does not undergo a change even measured at varied angles.
In addition, recent liquid crystal display devices are required to have improved tint appearance. In order to satisfy it, not only the Re or Rth of an optically transparent film such as a protective film of a polarizer or support of an optically-compensatory film in a visible region ranging from a wavelength of from 400 to 800 nm must be reduced but also fluctuations in Re or Rth depending on wavelength, that is, chromatic dispersion must be reduced.
Although cellulose acylate films having a decreased in-plane Re have so far been known, it was difficult to manufacture cellulose acylate films undergoing small changes in the Re at varied angles, that is, cellulose acylate films having a reduced Rth. There is an eager demand for the development of an optically isotropic and optically transparent film made of cellulose acylate having an in-plane Re adjusted to almost zero, and undergoing small fluctuations in retardation at varied angles, which means having an Rth adjusted to almost zero.
When cellulose acylate films are prepared, a compound called plastizer is usually added in order to improve the film forming performance. As the plastizier, trimester phosphates such as triphenyl phosphate and biphenyldiphenyl phosphate and phthalate esters are disclosed (for example, refer to Plastic Zairyo Koza, Vol. 17, published by Nikkan Kogyoshinbun, “Sen 'iso-kei Jushi”, p121 (1970)). Some of these plasticizers are known to have effects for lowering the optical anisotropy of cellulose acylate films, but the effects for reducing optical anisotropy of cellulose acylate films are not sufficient.
In JP-A-2005-41911, a cellulose acylate film which contains a phosphoric acid plasticizer having a specific structure is disclosed. This plastizier is also insufficient for lowering the optical anisotropy of cellulose acylate films.
Further, cellulose acylate films whose Rth has been reduced sufficiently by the addition thereto of a retardation controller or retardation regulator or cellulose acylate films having retardation reduced by using cellulose acylate having a high substitution degree are known, but effects for decreasing the chromatic dispersion of Rth are not sufficient.
In order to impart cellulose acylate films with various functions, various additives have been added thereto. One of the additives is an infrared absorber.
In JP-A-2001-194522, an optical film containing an infrared absorbing dye is disclosed and a technology capable of decreasing Re and Rth is proposed. In JP-2004-325523, proposed is a technology of using an infrared absorber and/or ultraviolet absorber as a retardation controller exhibiting dichroism and reducing, over a wide wavelength range, fluctuations of the Re of a film obtained by mixing the retardation controller in a resin having refractive index anisotropy. The conventional method using an infrared absorber has however a problem that when it is used for a liquid crystal display device, an effect of regulating the chromatic dispersion of the Rth is insufficient, or transparency is insufficient owing to unignorable coloring in a visible region.